Im going to be honest. We have a user that has Solidworks and I know zero about it. He has 2014/2016/2018 installed on his PC (just installed 2018 recently) but opening 2018 gives him some error about his PDM addin being old. What do I need to find out and how do I get him PDM that works with 2018?

I suspect that he has the PDM add in turned on and that is why it is calling for it to be updated. If he will clear the box shown below the error message should stop.

PDM standard, while it comes free with SW Pro and Premium still requires to be loaded from the setup disks that come with Solid Works. Then it needs to be setup (usually by your re-seller or someone trained to get it right) which is where the $$$ comes in. If you only have one user it isn't necessary, but even then would be helpful down the road so you might want to talk to your re-seller for more information. If you are just getting started with SolidWorks now would be the easiest time to get it set up.